The recent application of therapies using combinations of antiviral drugs has shown that virus growth in infected people can be brought to a halt and - in many individuals - provide considerable and long-lasting improvement in their condition. These therapies have helped large numbers of people live relatively normal lives despite their HIV infection. Most importantly, they prove the concept that antiviral drugs can give long-term relief to patients with HIV infection, but fall far short of providing a long-term solution. The problem facing all the strategies is the development of resistance in the virus due to the appearance of specific mutations. In an effort to avoid resistance, drugs have to be given at high - somewhat toxic - doses, in expensive combinations, and on exacting and difficult to follow schedules. Even then, the therapy often fails, and resistant virus appears. There is, therefore, a desperate need to understand how the virus develops resistance to drugs, and to use this understanding to develop more effective strategies for treating HIV infection. The goal of the DRP is to establish a focused basic science research effort that addresses this need and builds on the existing strength of HIV and retrovirus research within the National Cancer Institute.